CN111213267A

CN111213267A - Positive electrode active material for secondary battery, method of preparing the same, and lithium secondary battery comprising the same

Info

Publication number: CN111213267A
Application number: CN201880067132.2A
Authority: CN
Inventors: 金智惠; 郑王谟; 李东勋; 朴星彬; 金东晖; 曹炯万; 韩政珉
Original assignee: LG Chem Ltd
Current assignee: LG Energy Solution Ltd
Priority date: 2017-11-21
Filing date: 2018-11-08
Publication date: 2020-05-29
Anticipated expiration: 2038-11-08
Also published as: KR20190057951A; CN111213267B; EP3686971A4; WO2019103363A1; JP2020537315A; JP7086412B2; EP3686971A1; US20200259173A1; KR102268079B1

Abstract

The present invention relates to a positive electrode active material for a secondary battery, which includes a core portion and a shell portion formed around the core portion. Wherein the core section and the shell section contain a lithium composite transition metal oxide containing Ni and Co and at least one selected from the group consisting of Mn and Al, and a ratio of a diameter of the core section to a total diameter of the positive electrode active material particles is 0.5 to 0.85, and the shell section has a concentration gradient such that a Ni content at a start point of the shell section on the core section side is higher by 30 mol% or more than a Ni content at an end point of the shell section on the surface side of the particles.

Description

Positive electrode active material for secondary battery, method of preparing the same, and lithium secondary battery comprising the same

Cross Reference to Related Applications

The present application claims priority and benefit from korean patent application No. 10-2017-0155468, filed on 21/11/2017, the disclosure of which is incorporated herein by reference in its entirety.

Technical Field

The present invention relates to a positive electrode active material for a secondary battery, a method of preparing the same, and a lithium secondary battery including the same.

Background

Recently, with the rapid spread of electronic devices using batteries, such as mobile phones, notebook computers, and electric vehicles, the demand for secondary batteries having a small size and light weight and a higher capacity has rapidly risen. In particular, lithium secondary batteries are attracting attention as driving power sources for portable devices because of their small size and light weight and high energy density. As a result, research and development attempts have been actively made to improve the performance of the lithium secondary battery.

The lithium secondary battery includes an organic electrolyte or a polymer electrolyte filled between a positive electrode and a negative electrode (composed of an active material capable of intercalating and deintercalating lithium ions), and generates electric energy through oxidation and reduction when lithium ions are intercalated/deintercalated into/from the positive electrode and the negative electrode.

As a positive electrode active material of a lithium secondary battery, lithium cobalt oxide (LiCoO) has been used₂) Lithium nickel oxide (LiNiO)₂) Lithium manganese oxide (LiMnO)₂Or LiMn₂O₄) Or lithium iron phosphate (LiFePO)₄) And (c) a compound such as a quaternary ammonium compound. Further, LiNiO is a supporting substance₂A lithium composite metal oxide in which part of nickel (Ni) is substituted with cobalt (Co) or manganese (Mn)/aluminum (Al) (hereinafter, simply referred to as "NCM-based lithium composite transition metal oxide" or "NCA-based lithium composite transition metal oxide") has been developed. However, the previously developed NCM/NCA-based lithium composite transition metal oxide has a limitation in its application due to insufficient capacity characteristics.

In order to improve the above problems, recently, studies on increasing the Ni content in the NCM/NCA-based lithium composite transition metal oxide are being conducted. However, the high Ni cathode active material having a high Ni content has problems of reduced structural and chemical stability and rapidly reduced thermal stability as an active material.

Therefore, it is required to develop a high Ni positive electrode active material having a high capacity and excellent in structure, chemical stability and thermal stability.

Disclosure of Invention

[ problem ] to

The present invention is directed to provide a high Ni type lithium composite transition metal oxide positive electrode active material having a high Ni concentration, which has a high capacity, enhanced structural and chemical stability, and improved thermal stability.

[ solution ]

The present invention provides a positive electrode active material for a secondary battery, including a core part and a shell part formed around the core part, wherein the core part and the shell part include a lithium composite transition metal oxide containing Ni and Co and at least one selected from the group consisting of Mn and Al. Here, the ratio of the diameter of the core portion to the total diameter of the positive electrode active material particles is 0.5 to 0.85, and the shell portion has a concentration gradient such that the Ni content at the start of the shell portion on the core portion side is higher than the Ni content at the end of the shell portion on the particle surface side by 30 mol% or more.

Further, the present invention provides a method of preparing a positive electrode active material for a secondary battery, comprising: co-precipitating a first precursor-forming solution containing a Ni-containing raw material, a Co-containing raw material, and at least one selected from a Mn-containing raw material and an Al-containing raw material to form a core; co-precipitating a second precursor-forming solution containing a Ni-containing raw material, a Co-containing raw material, and at least one selected from a Mn-containing raw material and an Al-containing raw material to form a shell section, wherein a concentration of the Ni-containing raw material is lower than a concentration of the Ni-containing raw material in the first precursor-forming solution; the method includes forming a positive electrode active material precursor including a core portion and a shell portion formed around the core portion, mixing the positive electrode active material precursor with a lithium source, and calcining the mixture, thereby forming a positive electrode active material including a lithium composite transition metal oxide. Here, the ratio of the diameter of the core portion to the total diameter of the particles of the positive electrode active material is 0.5 to 0.85, and the shell portion has a concentration gradient such that the Ni content at the start of the shell portion on the core portion side is 30 mol% or more of the Ni content at the end of the shell portion on the particle surface side.

In addition, the present invention provides a positive electrode and a lithium secondary battery including the positive electrode active material.

[ advantageous effects ]

According to the present invention, as the Ni content is further increased, the lithium composite transition metal oxide may have high capacity, enhanced structural and chemical stability, and improved thermal stability.

A lithium secondary battery prepared using the positive electrode active material for a secondary battery of the present invention may have an enhanced charge/discharge capacity and enhanced battery characteristics, such as life characteristics.

Drawings

Fig. 1 is a graph of leakage current versus time for a lithium secondary battery (full cell) using the positive electrode prepared according to the example and comparative example.

Detailed Description

Hereinafter, the present invention will be described in more detail to help understanding of the present invention. The terms or words used in the specification and claims should not be construed as limited to general or dictionary meanings, but interpreted using meanings and concepts consistent with technical ideas of the present invention on the basis of the principle that the inventor appropriately defines the concepts of the terms to best explain the present invention.

The positive electrode active material for a secondary battery of the present invention includes a core part and a shell part formed around the core part, wherein the core part and the shell part include a lithium composite transition metal oxide containing Ni and Co and at least one selected from the group consisting of Mn and Al. Here, the ratio of the diameter of the core portion to the total diameter of the positive electrode active material particles is 0.5 to 0.85, and the shell portion has a concentration gradient such that the Ni content at the start of the shell portion on the core portion side is higher than the Ni content at the end of the shell portion on the particle surface side by 30 mol% or more.

The core portion of the positive electrode active material of the present invention may contain a high Ni type lithium composite transition metal oxide in which the Ni content accounts for 80 mol% or more of the total metal elements contained in the lithium composite transition metal oxide. More preferably, in the core portion, the Ni content may account for 88 mol% or more of the total metal elements contained in the lithium composite transition metal oxide. Since the core portion contains the high Ni type lithium composite transition metal oxide having an Ni content of 80 mol% or more, a high capacity can be ensured.

In the positive electrode active material of the present invention, since the ratio of the diameter of the core portion containing the high Ni-type lithium composite transition metal oxide having an Ni content of 80 mol% or more to the total diameter of the positive electrode active material particles is 0.5 to 0.85, a high capacity can be secured. The ratio of the diameter of the core to the total diameter of the positive electrode active material particles is preferably 0.55 to 0.80, more preferably 0.55 to 0.70. When the ratio of the diameter of the core to the total diameter of the positive electrode active material particles is less than 0.5, it may be difficult to secure high capacity, and when the ratio is greater than 0.85, structural and chemical stability may be reduced, thermal stability may be significantly reduced, stability at high voltage may not be secured, and life characteristics may be reduced.

The core may be: an NCM-based lithium composite transition metal oxide containing Ni, Co, and Mn, an NCA-based lithium composite transition metal oxide containing Ni, Co, and Al, or a 4-component type lithium composite transition metal oxide containing substantially four elements such as Ni, Co, Mn, and Al. Since the core contains Mn and/or Al and Ni and Co, structural stability can be greatly enhanced and life characteristics can be significantly improved. In general, when a lithium composite transition metal oxide contains Mn and/or Al, it may be effective in terms of stability, but is disadvantageous in terms of capacity characteristics and output characteristics. However, when the composition and structure of the cathode active material of the present invention are satisfied, the capacity characteristics can be sufficiently ensured, and the stability and life characteristics can be significantly improved.

The Ni concentration of the core may be constant, and the concentrations of Co, Mn, and Al contained may also be constant. That is, the core may be a lithium composite transition metal oxide having no concentration gradient.

As with the core, the shell portion of the positive electrode active material of the present invention may be: an NCM-based lithium composite transition metal oxide containing Ni, Co, and Mn, an NCA-based lithium composite transition metal oxide containing Ni, Co, and Al, or a 4-component type lithium composite transition metal oxide containing substantially four elements such as Ni, Co, Mn, and Al.

The shell portion has a concentration gradient of a metal component of the lithium composite transition metal oxide, specifically, such that the Ni concentration at the beginning of the shell portion on the core portion side is higher by 30 mol% or more than the Ni concentration at the end of the shell portion on the particle surface side. More preferably, the concentration gradient of the Ni concentration at the beginning of the shell portion may be such that it is higher than the Ni concentration at the end of the shell portion by 40 mol% or more, further preferably, the concentration gradient is such that it is higher than the Ni concentration at the end of the shell portion by 50 mol% or more. Although the cathode active material of the present invention is a lithium composite transition metal oxide having a very high Ni content, since the Ni concentration at the beginning of the shell section is higher than that at the end of the shell section by 30 mol% or more, structural and chemical stability can be improved, and thermal stability can be significantly improved.

In the present invention, the concentration gradient and concentration of the transition metal in the positive electrode active material may be identified using a method such as Electron Probe Microanalysis (EPMA), inductively coupled plasma atomic emission spectroscopy (ICP-AES), time-of-flight secondary ion mass spectrometry (ToF-SIMS), EDAX-SEM analysis, or X-ray photoelectron spectroscopy (XPS), and specifically, the atomic ratio of the metal may be measured using EPMA while moving from the center of the positive electrode active material to the surface thereof, or the atomic ratio of the metal may be measured by performing etching from the surface of the positive electrode active material to the center thereof and using XPS.

In an exemplary embodiment of the present invention, the shell portion may exhibit a concentration gradient such that the Ni concentration gradually decreases from a start point of the shell portion to an end point of the shell portion. Since the Ni concentration at the shell start point is kept at a high level and gradually decreases toward the shell end point on the surface side, thermal stability can be exhibited and a decrease in capacity can be prevented. In particular, even if the core having a very high Ni content is present at a diameter ratio of 0.5 to 0.85, since the concentration gradient of the shell portion causes the Ni concentration to gradually decrease such that it differs by 30 mol% or more between the start point and the end point of the shell portion, excellent stability can be exhibited.

Meanwhile, the shell portion may have a concentration gradient such that a concentration of at least one of Mn and Co gradually increases from a start point to an end point of the shell portion. In this case, since the Mn concentration at the start point of the shell section is kept at a low level and the Mn concentration increases toward the end point of the shell section on the surface side, excellent thermal stability can be obtained, and since the Co concentration at the start point of the shell section is kept at a low level and increases toward the end point of the shell section on the surface side, the amount of Co can be reduced, and also a capacity drop can be prevented.

The shell portion of the cathode active material according to the exemplary embodiment of the present invention has a continuous concentration gradient such that the Ni concentration in the shell portion decreases from the shell portion starting point toward the shell portion end point on the surface side, and has a continuous concentration gradient complementary to the Ni concentration gradient such that the concentration of at least one of Mn and Co increases from the shell portion starting point toward the shell portion end point on the surface side.

In the present invention, the "concentration gradient showing a gradual change (increase or decrease) in the concentration of the transition metal" means that the following concentration profile exists: the concentration of the transition metal varies gradually throughout the particle. Specifically, the concentration distribution shows that the transition metal concentration difference per 1 μm in the particles may be 0.1 to 5 mol%, more specifically 0.1 to 3 mol%, and still more specifically 1 to 2 mol%, based on the total number of moles of the corresponding metal contained in the positive electrode active material.

According to exemplary embodiments of the present invention, when the transition metal in the shell portion of the positive electrode active material has a concentration gradient in which the concentration continuously changes, there is no severe phase boundary region from the start point to the end point of the shell portion, and thus the crystal structure may be stabilized and the thermal stability may be further improved. In addition, when the slope of the concentration gradient of the transition metal is constant, the effect of improving the structural stability can be further enhanced.

The average Ni content of the shell portion of the positive electrode active material of the present invention may be 50 to 90 mol%, preferably 60 to 88 mol%, of the total metal elements contained in the lithium composite transition metal oxide. This is the average Ni composition of the entire shell portion. Although the Ni average content of the shell portion is high, for example, 50 mol% to 90 mol%, the Ni concentration may be maintained at a high level at the shell portion starting point, and since the Ni concentration decreases by 30 mol% or more toward the shell portion end point of the surface side, a decrease in capacity may be prevented, and excellent thermal stability may be exhibited.

In the case of the cathode active material according to the exemplary embodiment of the present invention, the ratio of the thickness of the shell portion to the particle radius of the cathode active material may be 0.15 to 0.5, preferably 0.2 to 0.45, and more preferably 0.3 to 0.45. When the ratio of the thickness of the shell portion to the particle radius of the cathode active material is less than 0.15, structural and chemical stability is reduced, thermal stability is significantly reduced, stability at high voltage cannot be ensured, and life characteristics are deteriorated. When the ratio of the thickness of the shell portion to the particle radius of the cathode active material is greater than 0.5, it may be difficult to secure a high capacity.

Meanwhile, the shell portion of the cathode active material according to the exemplary embodiment of the present invention may include the following lithium composite transition metal oxide particles: which has a crystal orientation that grows radially in a direction from the center of the positive electrode active material particle to the surface thereof. By satisfying the structures of the core section and the shell section, the structural stability of the positive electrode active material can be further improved, and the output characteristics can be further improved.

The core and shell portions of the positive electrode active material of the present invention may include a lithium composite transition metal oxide represented by formula 1.

[ formula 1]

Li_pNi_1-(x1+y1+z1)Co_x1M^a _y1M^b _z1M^c _q1O₂

In this formula, M^aIs at least one element selected from the group consisting of Mn and Al, M^bIs at least one element selected from the group consisting of Zr, W, Mg, Al, Ce, Hf, Ta, La, Ti, Sr, Ba, Nb, Mo and Cr, M^cIs at least one element selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr, p is 0.9-1.5, 0<x1≤0.4，0<y1≤0.4，0≤z1≤0.1，0≤q1≤0.1，0<x1+y1+z1≤0.4。

The composition of the lithium composite transition metal oxide of formula 1 is the total average composition of the core and shell portions. That is, the cathode active material according to an exemplary embodiment of the present invention may be composed of a core portion including the lithium composite transition metal oxide of formula 1 and a shell portion including the lithium composite transition metal oxide having the average composition of formula 1.

In the lithium composite transition metal oxide of formula 1, the content of Li may be p, i.e., 0.9. ltoreq. p.ltoreq.1.5. When p is less than 0.9, there is a possibility of capacity reduction, and when p is more than 1.5, it may be difficult to prepare a cathode active material because the particles are calcined during calcination. In view of the balance between the significant improvement of the capacity characteristics of the positive electrode active material due to the control of the Li content and the sinterability in the preparation of the active material, it is preferable that the Li content be 1.0. ltoreq. p.ltoreq.1.15.

In the lithium composite transition metal oxide of formula 1, the content of Ni may correspond to 1- (x1+ y1+ z1), which satisfies, for example, 0.6 ≦ 1- (x1+ y1+ z1) < 1. When the Ni content in the lithium composite transition metal oxide of formula 1 is 0.6 or more, a sufficient Ni amount contributing to charge and discharge can be secured to achieve a high capacity. More preferably, the Ni content may be 0.8. ltoreq.1- (x1+ y1+ z 1). ltoreq.0.99. More specifically, the Ni content in the core portion may be 0.8. ltoreq.1- (x1+ y1+ z 1). ltoreq.0.99, and the average Ni content in the shell portion may be 0.6. ltoreq.1- (x1+ y1+ z 1). ltoreq.0.9.

In the lithium composite transition metal oxide of formula 1, the Co content may be x1, i.e., 0< x1 ≦ 0.4. When the Co content in the lithium composite transition metal oxide of formula 1 is greater than 0.4, costs may be increased. In view of the remarkable improvement in capacity characteristics due to the inclusion of Co, the Co content is more specifically 0.05. ltoreq. x 1. ltoreq.0.2.

In the lithium composite transition metal oxide of formula 1, M^aWhich may be Mn or Al, or Mn and Al, may enhance the stability of the active material, resulting in improved battery stability. Considering the effect of improving the life characteristics, M^aMay be y1, i.e. 0<y1 is less than or equal to 0.4. When y1 of the lithium composite transition metal oxide of formula 1 is greater than 0.4, the output characteristics and capacity characteristics of the battery may be deteriorated, and M^aThe content of (b) may be more specifically 0.05. ltoreq. y 1. ltoreq.0.2.

In the lithium composite transition metal oxide of formula 1, M^bMay be a doping element, M, contained in the crystal structure of the lithium composite transition metal oxide^bThe content of (b) can be z1, i.e. 0. ltoreq. z 1. ltoreq.0.1.

In the lithium composite transition metal oxide of formula 1, the metal element M^cMay not be contained in the structure of the lithium composite transition metal oxide, and may be prepared as follows^cLithium composite transition metal oxide of (2): adding the precursor and the lithium source and adding M during the mixing and calcining of the precursor and the lithium source^cThe source is calcined together with the precursor and the lithium source, or a lithium composite transition metal oxide is formed and then mixed with M added separately^cThe sources are calcined together. Without deteriorating the characteristics of the positive electrode active material, M^cThe content of (b) may be q1, i.e. 0. ltoreq. q 1. ltoreq.0.1.

In addition, the cathode active material according to the exemplary embodiment of the present invention may further include a surface layer formed on an outer circumference of the shell portion. The surface layer may include a lithium composite transition metal oxide including at least one selected from the group consisting of Ni, Co, Mn, and Al. The concentration of the transition metal in the surface layer may be constant.

In addition, the positive electrode active material may be composed of secondary particles in which primary particles of the lithium composite transition metal oxide are aggregated, and a lithium ion diffusion path in the primary particles may be formed toward the center direction of the secondary particles. That is, the a or b axis in the layered structure of the lithium composite transition metal oxide may be formed in the central direction of the secondary particle. Accordingly, adsorption and release of lithium ions are promoted, which may be advantageous in terms of capacity characteristics and output characteristics.

Hereinafter, a method of preparing the positive electrode active material for a secondary battery of the present invention will be described.

The method for preparing the positive active material for a secondary battery of the present invention comprises: co-precipitating a first precursor-forming solution containing a Ni-containing raw material, a Co-containing raw material, and at least one selected from the group consisting of a Mn-containing raw material and an Al-containing raw material to form a core portion, and Co-precipitating a second precursor-forming solution containing a Ni-containing raw material, a Co-containing raw material, and at least one selected from the group consisting of a Mn-containing raw material and an Al-containing raw material to form a shell portion, wherein a concentration of the Ni-containing raw material is lower than a concentration of the Ni-containing raw material in the first precursor-forming solution; the method includes forming a positive electrode active material precursor including a core portion and a shell portion formed around the core portion, mixing the positive electrode active material precursor with a lithium source, and calcining the mixture, thereby forming a positive electrode active material including a lithium composite transition metal oxide. Here, the ratio of the diameter of the core portion to the total diameter of the positive electrode active material particles is 0.5 to 0.85, and the shell portion has a concentration gradient such that the Ni concentration at the start point of the shell portion on the core portion side is higher than the Ni concentration at the end point of the shell portion on the particle surface side by 30 mol% or more.

To prepare the positive electrode active material of the present invention, first, a positive electrode active material precursor including a core section and a shell section is formed. The core portion is formed by coprecipitating a first precursor-forming solution (which contains a Ni-containing raw material, a Co-containing raw material, and at least one selected from the group consisting of a Mn-containing raw material and an Al-containing raw material), and the shell portion is formed by further adding a second precursor-forming solution that contains a Ni-containing raw material and has a lower concentration than the first precursor-forming solution and performing the coprecipitation.

The Ni-containing raw material, the Co-containing raw material, the Mn-containing raw material, and the Al-containing raw material may be a sulfate, a halide, an acetate, a sulfide, a hydroxide, an oxide, or a oxyhydroxide containing Ni, Co, Mn, or Al, respectively, but are not particularly limited thereto as long as they are soluble in water. For example, the Ni-containing feedstock may be Ni (SO)₄)₂·7H₂O、NiSO₄、NiCl₂、Ni(OH)₂Or Ni (OCOCH)₃)₂·4H₂O or Ni (NO)₃)₂·6H₂O, any one or a mixture of two or more thereof may be used.

The preparation of the first and second precursor-forming solutions may be prepared as follows: adding a Ni-containing raw material, a Co-containing raw material, a Mn-containing raw material, and an Al-containing raw material to a solvent, specifically, water, or a mixture of water and an organic solvent (specifically, alcohol, etc.) that can be uniformly mixed with water; or preparing solutions containing the respective raw materials and then mixing these solutions together.

The positive active material precursor may be prepared by adding a precursor-forming solution to a reactor, and adding a chelating agent and an aqueous alkali solution, and performing co-precipitation.

The chelating agent may be NH₄OH、(NH₄)₂SO₄、NH₄NO₃、NH₄Cl、CH₃COONH₄Or NH₄CO₃One or a mixture of two or more thereof may be used. Further, the chelating agent may be used in the form of an aqueous solution, and the solvent may be water, or a mixture of water and an organic solvent (specifically, alcohol, etc.) which can be uniformly mixed with water.

The basic compound may be an alkali or alkaline earth metal hydroxide, for example NaOH, KOH or Ca (OH)₂Or a hydrate thereof, one or both of which may be usedMixtures of the above. The basic compound may also be used in the form of an aqueous solution, and the solvent may be water, or a mixture of water and an organic solvent (specifically, alcohol, etc.) which can be uniformly mixed with water. Here, the concentration of the alkaline aqueous solution may be 2M to 10M.

The co-precipitation for preparing the positive electrode active material precursor may be performed under the condition of pH 10 to 12. When the pH is outside the above range, the size of the prepared positive electrode active material precursor may be changed or the particles may be cracked. More specifically, the coprecipitation may be performed at pH 11 to 12. The pH can be controlled by adding an aqueous alkaline solution.

The co-precipitation for preparing the positive electrode active material precursor may be performed at a temperature ranging from 30 ℃ to 80 ℃ under an inert atmosphere such as nitrogen. In order to increase the reaction rate in the reaction, a stirring process may be optionally performed, and the stirring speed may be 100rpm to 2000 rpm.

By increasing the concentration of the Ni-containing raw material of the first precursor-forming solution, the core portion can be formed so that the Ni content thereof accounts for 80 mol% or more, more preferably 88 mol% or more of the total metal elements.

Here, a high Ni core having an Ni content of 80 mol% or more may be formed, and the ratio of the diameter thereof to the total particle diameter of the positive electrode active material precursor is 0.5 to 0.85. The diameter ratio of the core may be adjusted by controlling the coprecipitation time for forming the positive active material precursor. The co-precipitation time for forming the core may be 0.5 to 0.85 times, more preferably 0.6 to 0.8 times, the total precipitation time for forming the positive electrode active material precursor. Specifically, the total precipitation time for forming the positive electrode active material precursor may be 24 hours to 40 hours, wherein the coprecipitation time for forming the core may be 13 hours to 32 hours, and more preferably 15 hours to 25 hours.

Coprecipitation is performed by further adding a second precursor-forming solution, the concentration of which is lower than that of the first precursor-forming solution, to form a shell section having a concentration gradient such that the Ni concentration at the beginning of the shell section is 30 mol% or more higher than that at the end of the shell section. For example, the molar ratio of the Ni salt of the first precursor-forming solution to the total transition metal salt may be 80 mol% or more, and the molar ratio of the Ni salt of the second precursor-forming solution to the total transition metal salt may be less than 80 mol%.

The shell portion may be formed to have a concentration gradient in which the Ni concentration gradually decreases from a start point to an end point of the shell portion.

Meanwhile, the second precursor forming solution may include at least one of raw materials of Mn and Co at a higher concentration than the first precursor forming solution. The shell portion formed by performing the Co-precipitation by further adding the second precursor-forming solution may have a concentration gradient such that the concentration of at least one of Mn and Co gradually increases from a start point to an end point of the shell portion.

The positive active material precursor prepared by the co-precipitation may be separated according to a conventional method, and then a drying process may be selectively performed at 110 to 400 ℃ for 15 to 30 hours.

Then, a positive electrode active material precursor is formed as described above, and then mixed with a lithium source, and the mixture is calcined, thereby forming a positive electrode active material including a lithium composite transition metal oxide.

The lithium source may be a sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide or oxyhydroxide containing lithium, and is not particularly limited as long as it is soluble in water. Specifically, the lithium raw material may be Li₂CO₃、LiNO₃、LiNO₂、LiOH、LiOH·H₂O、LiH、LiF、LiCl、LiBr、LiI、CH₃COOLi、Li₂O、Li₂SO₄、CH₃COOLi or Li₃C₆H₅O₇Any one or a mixture of two or more thereof may be used.

Meanwhile, when the precursor is mixed with the lithium source, a calcinator may be optionally added. Specifically, the calcining agent may be: compounds containing ammonium ions, e.g. NH₄F、NH₄NO₃Or (NH)₄)₂SO₄(ii) a Metal oxides, e.g. B₂O₃Or Bi₂O₃(ii) a Or metal halides, e.g. NiCl₂Or CaCl₂Any one or a mixture of two or more thereof may be used. The content of the calcination agent may be 0.01 to 0.2 mol with respect to 1 mol of the precursor.

In addition, a water scavenger may be optionally added when the precursor is mixed with the lithium source. Specifically, the water scavenger may be citric acid, tartaric acid, glycolic acid, or maleic acid, and any one or a mixture of two or more thereof may be used. The water scavenger may be contained in an amount of 0.01 to 0.2 mol with respect to 1 mol of the precursor.

The calcination process may be performed at 700 ℃ to 900 ℃, more preferably at 750 ℃ to 880 ℃. When the calcination temperature is less than 700 deg.c, the discharge capacity, the cycle characteristic, and the operating voltage per unit weight may be deteriorated due to the remaining unreacted raw materials, and when the calcination temperature is more than 900 deg.c, the discharge capacity, the cycle characteristic, and the operating voltage per unit weight may be deteriorated due to the generation of by-products.

The calcination process may be performed in an oxidative atmosphere such as air or oxygen, or in a reductive atmosphere containing nitrogen or hydrogen for 5 to 30 hours.

The ratio of the core portion diameter of the positive electrode active material prepared as described above to the total diameter of the particles of the positive electrode active material may be 0.5 to 0.85, and the shell portion has a concentration gradient such that the Ni concentration at the beginning of the shell portion on the core portion side is higher than the Ni concentration at the end of the shell portion on the particle surface side by 30 mol% or more. The Ni content of the core may be 80 mol% or more, and more preferably 88 mol% or more, of the total metal elements contained in the lithium composite transition metal oxide.

The ratio of the core diameter of the positive electrode active material to the total diameter of the positive electrode active material particles may be 0.5 to 0.85, and the ratio of the shell thickness to the positive electrode active material particle radius may be 0.15 to 0.5.

Since the positive electrode active material of the present invention has a high Ni content, a high capacity can be achieved, structural stability and chemical stability can be improved, and excellent thermal stability can be ensured.

According to another exemplary embodiment of the present invention, there are provided a positive electrode for a lithium secondary battery and a lithium secondary battery including the positive electrode active material prepared as above.

Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer containing the positive electrode active material formed on the positive electrode current collector.

In the positive electrode, the positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity, and may be stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium or silver. The thickness of the positive electrode current collector may be generally 3 to 500 μm, and the adhesive strength of the positive electrode active material may be improved by forming fine irregularities on the surface of the positive electrode current collector. For example, the positive electrode collector may be used in various forms, such as a film, a sheet, a foil, a mesh, a porous body, a foam, a non-woven fabric, and the like.

Further, the positive electrode active material layer may include a conductive material and a binder in addition to the above-described positive electrode active material.

Here, the conductive material is used to provide conductivity to the electrode, and is not particularly limited as long as it has electronic conductivity and does not cause chemical changes in the battery. Specific examples of the conductive material may include: graphite, such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber composed of copper, nickel, aluminum or silver; conductive whiskers such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides such as titanium dioxide; or a conductive polymer such as a polyphenylene derivative, and one of them or a mixture of two or more of them may be used. The content of the conductive material may be generally 1 to 30% by weight, based on the total weight of the positive electrode active material layer.

In addition, the binder serves to enhance bonding between the positive electrode active material particles and adhesion between the positive electrode active material and the positive electrode current collector. Specific examples of the binder may be polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of them or a mixture of two or more of them may be used. The binder may be contained in an amount of 1 to 30 wt% based on the total weight of the positive electrode active material layer.

The positive electrode may be prepared according to a conventional positive electrode preparation method, except for using the positive electrode active material. Specifically, the positive electrode can be manufactured by: the composition for forming a positive electrode active material layer, which includes the positive electrode active material and optionally a conductive agent and a binder, is coated on a positive electrode current collector, and then the composition is dried and rolled. Here, the kinds and contents of the positive electrode active material, the binder, and the conductive material are as described above.

The solvent may be a solvent commonly used in the art, and may be dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of them or a mixture of two or more of them may be used. The solvent is used in an amount sufficient to dissolve or disperse the positive electrode active material, the conductive material, and the binder, and then has a viscosity showing excellent thickness uniformity when coating is performed to prepare the positive electrode, in consideration of the coating thickness of the slurry and the manufacturing yield.

As another method, the positive electrode can be manufactured as follows: the composition for forming a positive electrode active material layer is cast on a separate support, and a film obtained by peeling from the support is laminated on a positive electrode current collector.

According to still another exemplary embodiment of the present invention, there is provided an electrochemical device including the positive electrode. The electrochemical device may be embodied as a battery or a capacitor, more specifically, a lithium secondary battery.

The lithium secondary battery specifically includes a positive electrode, a negative electrode disposed opposite to the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is as described above. In addition, the lithium secondary battery may selectively include: a battery case accommodating an electrode assembly including a positive electrode, a negative electrode, and a separator, and a sealing member for sealing the battery case.

In the lithium secondary battery, the anode includes an anode current collector and an anode active material layer on the anode current collector.

The negative electrode collector is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery, and may be, for example: copper, stainless steel, aluminum, nickel, titanium, fired carbon, or copper or stainless steel surface treated with carbon, nickel, titanium or silver, or aluminum cadmium alloys. In addition, the thickness of the negative electrode current collector may be generally 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to enhance the adhesive strength of the negative electrode active material, as with the positive electrode current collector. For example, the negative electrode current collector may be used in various forms, such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, and the like.

The negative electrode active material layer may optionally include a binder and a conductive material in addition to the negative electrode active material. For example, the anode active material layer may be prepared as follows: the negative electrode forming composition (which contains a negative electrode active material and optionally a binder and a conductive material) is coated on a negative electrode current collector and dried, or the negative electrode forming composition is cast on a separate support and a film obtained by peeling off the support is laminated on the negative electrode current collector.

As the negative electrode active material, a compound capable of reversibly intercalating and deintercalating lithium may be used. Specific examples of the anode active material may be: carbon-based materials such as artificial graphite, natural graphite, graphitized carbon fiber, or amorphous carbon; a metal compound alloyable with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; metal oxides capable of doping and dedoping lithium, e.g. SiO_β(0<β<2)、SnO₂Vanadium oxide and lithium vanadium oxide; or a composite containing a metal compound and a carbon-based material, such as a Si — C composite or a Sn — C composite, and either one or a mixture of two or more thereof may be used. In addition, a metallic lithium thin film may be used as a negative electrode active material. Further, both low crystalline carbon and high crystalline carbonCan be used as a carbon material. Representative examples of the low crystalline carbon include soft carbon and hard carbon, and representative examples of the high crystalline carbon include irregular, planar, flaky, spherical or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microbeads, mesophase pitch and high-temperature sintered carbon (e.g., petroleum tar or pitch-derived coke).

In addition, the binder and the conductive agent may be the same as described above for the positive electrode.

Meanwhile, in the lithium secondary battery, the separator is not particularly limited as long as it is commonly used to separate the anode and the cathode and provide a moving path for lithium ions in the lithium secondary battery, and in particular, the separator has low resistance to movement of electrolyte ions and is excellent in the ability to permeate an electrolyte. Specifically, a porous polymer film such as a porous polymer film made of a polyolefin-based polymer (e.g., an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer), or a laminated structure containing two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, such as a nonwoven fabric formed of high-melting glass fibers or polyethylene terephthalate fibers, may be used. In addition, the coated separator including a ceramic component or a polymer material may be used to ensure heat resistance or mechanical strength, and may be selectively used in a single layer or a multi-layer structure.

In addition, the electrolyte used in the present invention may be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a melt-type inorganic electrolyte that may be used to manufacture a lithium secondary battery, but the present invention is not limited thereto.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

The organic solvent is not particularly limited as long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, the organic solvent may be: ester solvents such as methyl acetate, ethyl acetate, γ -butyrolactone and ∈ -caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene or fluorobenzene; a carbonate-based solvent such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), Ethyl Methyl Carbonate (EMC), Ethylene Carbonate (EC), or Propylene Carbonate (PC); alcoholic solvents, such as ethanol or isopropanol; nitrile solvents, such as R — CN (where R is a linear, branched, or cyclic C2 to C20 hydrocarbyl group, and may include double-bonded aromatic rings or ether linkages); amide solvents such as dimethylformamide; dioxolane-based solvents such as 1, 3-dioxolane; or sulfolane solvents. Among these solvents, it is preferable to use a carbonate-based solvent, and it is more preferable to use a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant, which improves the charge/discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethylene carbonate, dimethyl carbonate, or diethyl carbonate). In this case, when a mixture of cyclic carbonate and chain carbonate in a volume ratio of about 1:1 to about 1:9 is used, the electrolyte may exhibit excellent performance.

The lithium salt is not particularly limited as long as it is a compound capable of providing lithium ions used in the lithium secondary battery. Specifically, the lithium salt may be LiPF₆、LiClO₄、LiAsF₆、LiBF₄、LiSbF₆、LiAlO₄、LiAlCl₄、LiCF₃SO₃、LiC₄F₉SO₃、LiN(C₂F₅SO₃)₂、LiN(C₂F₅SO₂)₂、LiN(CF₃SO₂)₂LiCl, LiI or LiB (C)₂O₄)₂. The concentration of the lithium salt is preferably 0.1M to 2.0M. When the concentration of the lithium salt is included in the above range, the electrolyte has appropriate conductivity and viscosity, and thus can exhibit excellent electrolytic performance. Therefore, lithium ions can be efficiently transferred.

In order to enhance the life characteristics of the battery, suppress the decrease in the capacity of the battery, and improve the discharge capacity of the battery, the electrolyte may further include one or more additives such as halogenated alkylene carbonate compounds (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, N-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinone, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, in addition to the above-mentioned electrolyte components. At this time, the content of the additive may be 0.1 to 5 wt% based on the total weight of the electrolyte.

Since the lithium secondary battery including the positive electrode active material of the present invention stably exhibits excellent discharge capacity, excellent output characteristics, and a high capacity retention rate (%), it can be used in the fields of portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as Hybrid Electric Vehicles (HEVs).

Therefore, according to still another exemplary embodiment of the present invention, there are provided a battery module (battery module) including the lithium secondary battery as a unit cell, and a battery pack (battery pack) including the battery module.

The battery module or the battery pack may be used as a power source for any one or more of medium to large-sized devices, including: an electric tool; electric motor vehicles, such as Electric Vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and an energy storage system.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily practice the invention. However, the present invention may be embodied in various forms and is not limited to the embodiments described herein.

Example 1

NiSO was introduced into a 5L batch reactor set at 60 deg.C₄、CoSO₄And MnSO₄Mixing in water such that a nickel to cobalt to manganese molar ratio is 95:4:1 to prepare a 2M first precursor forming solution (S1); mixing NiSO₄、CoSO₄And MnSO₄Mixing in water such that the nickel to cobalt to manganese molar ratio is 40:30:30 to prepare a 2M second precursor forming solution (S2).

1L of deionized water was charged into a coprecipitation reactor (volume: 5L), and the reactor was purged with nitrogen gas at a rate of 2L/min to remove oxygen dissolved in water and create a non-oxidizing atmosphere in the reactor. Then, 10mL of 25% aqueous NaOH solution was added and the solution was stirred at a stirring speed of 1200rpm at 60 ℃ to maintain the pH at 12.0.

Then, the first precursor forming solution was added at a rate of 180mL/hr, while simultaneously adding aqueous NaOH solution and NH₄Co-precipitation was performed for 18 hours while the OH aqueous solution was being performed, thereby forming a core. Then, the second precursor forming solution was added at 150mL/hr to perform coprecipitation for 12 hours, thereby forming a shell portion.

The thus-formed nickel-manganese-cobalt-based composite metal hydroxide particles composed of the core and shell portions were separated, cleaned, and dried in an oven at 120 deg.c, thereby preparing a positive electrode active material precursor.

The positive electrode active material precursor prepared as above and a lithium source LiOH were added to a henschel mixer (700L) and mixed at 300rpm for 20 minutes. The mixed powder was charged into an alumina furnace having a size of 330mm X330 mm in oxygen (O)₂) And calcined at 780 ℃ for 21 hours in an atmosphere, thereby preparing a positive active material of a lithium composite transition metal oxide.

Example 2

A positive electrode active material was prepared in the same manner as described in example 1, except that NiSO was used₄、CoSO₄And MnSO₄Mixing in water so that the molar ratio of nickel to cobalt to manganese was 90:5:5 to obtain a 2M first precursor forming solution (S1), and performing co-precipitation with the first precursor forming solution (S1) for 15 hours, thereby forming a core.

Comparative example 1

A positive electrode active material was prepared in the same manner as described in example 1, except that NiSO was used₄、CoSO₄And MnSO₄Mixing in water so that a nickel-cobalt-manganese molar ratio is 86:10:4 to obtain a 2M first precursor forming solution (S1), and performing co-precipitation using only the first precursor forming solution (S1) for 25 hours, thereby forming a positive electrode active material precursor, and preparing a positive electrode active material using the same.

Comparative example 2

A positive electrode active material was prepared in the same manner as described in example 1, except that a mixture of NiSO in a nickel: cobalt: manganese molar ratio of 70:10:20 was used₄、CoSO₄And MnSO₄The second precursor of (a) forms a solution (S2).

Comparative example 3

A cathode active material was prepared in the same manner as described in example 1, except that the first precursor forming solution (S1) was co-precipitated for 10 hours to form a core portion, and the second precursor forming solution (S2) was added at 150ml/hr and co-precipitated for 15 hours to form a shell portion.

Comparative example 4

A positive electrode active material was prepared in the same manner as described in example 1, except that the first precursor-forming solution (S1) was co-precipitated for 10 hours to form a core, and NiSO mixed with a molar ratio of nickel to cobalt to manganese of 70:10:20 was added at 180mL/hr₄、CoSO₄And MnSO₄Is co-precipitated for 15 hours to form the shell portion (S2).

Comparative example 5

A positive electrode active material was prepared in the same manner as described in example 1, except that NiSO was mixed in a nickel-cobalt molar ratio of 90:10₄And CoSO₄The first precursor-forming solution (S1) was coprecipitated for 23 hours to form a core section, and the second precursor-forming solution (S2) was coprecipitated for 2 hours to form a shell section.

[ Experimental example 1: analysis of Positive electrode active Material

In order to confirm the proportions of the core and shell portions and the Ni concentration difference in the shell portions of the positive electrode active materials prepared in examples 1 and 2 and comparative examples 1 to 5, each of the positive electrode active materials prepared according to examples and comparative examples was ion-milled to give a particle with a cross section, and the composition from the surface to the center of the particle was identified by an Electron Probe Microanalyzer (EPMA) analysis method. The results are shown in Table 1.

[ Table 1]

In table 1, the ratio of the core portion indicates the ratio of the diameter of the core portion to the diameter of the entire particle, and the ratio of the shell portion indicates the ratio of the thickness of the shell portion to the radius of the particle. Referring to table 1, in examples 1 and 2, the ratio of the core portion with respect to the diameter of the entire particle was 0.5 to 0.85, and the difference in Ni concentration between the starting point of the shell portion and the end point of the shell portion was 30 mol% or more. The shell portions in each of example 1 and example 2 had a concentration gradient such that the Ni concentration gradually decreased from the start point to the end point of the shell portion.

Meanwhile, comparative example 1 shows that the whole particles were made of LiNi of the same composition_0.86Co_0.1Mn_0.04O₂Comparative example 2, which is a positive electrode active material obtained as prepared, shows that the difference in Ni concentration between the beginning point and the end point of the shell portion is less than 30 mol%. In comparative examples 3 and 4, the proportion of the core portion was less than 0.5, and in comparative example 4, the difference in Ni concentration between the starting point and the end point of the shell portion was less than 30 mol%. In comparative example 5, the ratio of the core portion was greater than 0.85, and the ratio of the shell portion was 0.1.

[ Experimental example 2: evaluation of Charge/discharge Capacity and leakage Current

Each of the positive electrode active materials, the carbon black conductive material, and the PVdF binder prepared according to examples 1 and 2 and comparative examples 1 to 5 was mixed in N-methylpyrrolidone at a weight ratio of 96.5:1.5:2 to prepare a positive electrode mixture (viscosity: 5000mPa · s), which was applied to one surface of an aluminum current collector, dried at 130 ℃, and rolled to prepare a positive electrode. As the negative electrode, lithium metal was used.

A separator formed of porous polyethylene was disposed between the cathode and the anode formed as above to prepare an electrode assembly, the electrode assembly was placed in a case, and an electrolyte solution was injected into the case, thereby preparing a lithium secondary battery. Here, the electrolyte solution was prepared as follows: 1.0M lithium hexafluorophosphate (LiPF)₆) Dissolved in an organic solvent composed of ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate (EC/DMC/EMC mixed volume ratio: 3/4/3).

Charge/discharge experiments were performed on each lithium secondary battery (half cell) manufactured using each of the positive electrode active materials prepared in examples 1 and 2 and comparative examples 1 to 5. Specifically, charge/discharge was performed at 0.1C/0.1C at 50 ℃, and then charge was performed at 0.1C in the CCCV mode until the voltage was 4.7V, and was set to terminate after 130 hours. Meanwhile, the leakage current was measured for 130 hours, and the results are shown in table 2 below and fig. 1.

[ Table 2]

	Charge/discharge Capacity (mAh/g)	Average leakage Current (mAh/hr,130hr)
			Example 1	232/207	0.00
Example 2	230/205	0.06
			Comparative example 1	229/203	0.46
Comparative example 2	231/207	0.09
			Comparative example 3	229/204	0.25
Comparative example 4	230/205	0.32
			Comparative example 5	232/207	0.55

Referring to table 2 and fig. 1, it can be confirmed that in examples 1 and 2, the core portion and the shell portion both include Ni, Co and Mn, the ratio of the total diameter of the core portion to the particle is 0.5 to 0.85, the difference in Ni concentration between the start point and the end point of the shell portion is 30 mol% or more, excellent charge/discharge capacity is exhibited, and leakage current is hardly generated.

On the other hand, it can be seen that the capacity of comparative example 1 is slightly decreased and the leakage current at 130 hours is significantly increased, as compared with examples 1 and 2. In comparative example 1, since there was no shell portion having a difference in Ni concentration of 30 mol%, the stability was lowered. Further, although the capacity of comparative example 2 is the same as those of examples 1 and 2, referring to fig. 1, comparative example 2 has a higher leakage current as a whole than examples 1 and 2. This is because, in comparative example 2, since the Ni concentration difference in the shell portion was less than 30 mol%, sufficient stability could not be ensured. Further, in comparative examples 3 and 4, the ratio of the core was less than 0.5, the charge/discharge capacity was decreased, and the leakage current at 130 hours was significantly increased, as compared with examples 1 and 2. Further, in comparative example 5, since the core portion contained only Ni and Co and the ratio of the core portion was greater than 0.85, the charge/discharge capacity increase was large, but the leakage current at 130 hours was significantly increased.

[ Experimental example 3: evaluation of Life characteristics

Similar to experimental example 2, each lithium secondary battery (half cell) was manufactured using each of the cathode active materials prepared in examples 1 and 2 and comparative examples 1 to 5, and then subjected to 30 charge/discharge cycles to measure a capacity retention rate [% ], wherein each cycle included charging to 0.33C and 4.25V in CCCV mode at 45 ℃, cut-off at 0.005C, and discharging to 2.5V at a constant current of 0.33C. The results are shown in Table 3.

[ Table 3]

	Capacity retention (%) (45 ℃, 30 cycles)
		Example 1	96.0
Example 2	96.8
		Comparative example 1	95.2
Comparative example 2	94.0
		Comparative example 3	96.5
Comparative example 4	96.1
		Comparative example 5	89.0

Referring to table 3, examples 1 and 2 (in which the core section and the shell section each contain Ni, Co, and Mn, the ratio of the core section to the total diameter of the particle is 0.5 to 0.85, and the difference in Ni concentration between the start point of the shell section and the end point of the shell section is 30 mol% or more) exhibited very excellent high-temperature life characteristics.

On the other hand, the life characteristics of comparative examples 1 and 2 were reduced as compared with examples 1 and 2. In comparative example 1, since the shell portion having a Ni concentration difference of 30 mol% was not formed, the stability was lowered, and in comparative example 2, since the Ni concentration difference of the shell portion was less than 30 mol%, sufficient stability could not be secured. Further, in the case of comparative example 5 (in which the core portion contains only Ni and Co and the ratio of the core portion is greater than 0.85), the life characteristics were remarkably reduced as compared with examples 1 and 2. This is because structural stability is reduced because Mn/Al is not present in the core portion, and sufficient surface stability and thermal stability cannot be ensured because the shell portion has a small thickness.

Claims

1. A positive electrode active material for a secondary battery, comprising:

a core portion and a shell portion formed around the core portion,

wherein the core portion and the shell portion comprise a lithium composite transition metal oxide comprising: ni and Co, and at least one selected from the group consisting of Mn and Al,

a ratio of a diameter of the core to a total diameter of the particles of the positive electrode active material is 0.5 to 0.85, and

the shell section has a concentration gradient such that the Ni content at the beginning of the shell section on the core section side is higher by 30 mol% or more than the Ni content at the end of the shell section on the surface side of the particle.

2. The positive electrode active material according to claim 1, wherein the Ni content in the core portion is 80 mol% or more of all metal elements contained in the lithium composite transition metal oxide.

3. The positive electrode active material according to claim 1, wherein the Ni content in the core portion is 88 mol% or more of all metal elements contained in the lithium composite transition metal oxide.

4. The positive electrode active material according to claim 1, wherein the Ni concentration in the core portion is constant.

5. The cathode active material according to claim 1, wherein a concentration gradient of the shell portion is such that the Ni concentration gradually decreases from a start point of the shell portion to an end point of the shell portion.

6. The positive electrode active material according to claim 1, wherein in the shell portion, the Ni content accounts for 50 mol% to 90 mol% of all metal elements contained in the lithium composite transition metal oxide.

7. The cathode active material according to claim 1, wherein the shell portion includes lithium composite transition metal oxide particles having a crystal orientation that grows radially in a direction from a center to a surface of the particles of the cathode active material.

8. The cathode active material according to claim 1, wherein a ratio of a thickness of the shell portion to a radius of particles of the cathode active material is 0.15 to 0.5.

9. The cathode active material according to claim 1, wherein the core portion and the shell portion comprise a lithium composite transition metal oxide represented by the following formula 1:

[ formula 1]

Li_pNi_1-(x1+y1+z1)Co_x1M^a _y1M^b _z1M^c _q1O₂

Wherein M is^aIs at least one element selected from the group consisting of Mn and Al, M^bIs at least one element selected from the group consisting of Zr, W, Mg, Al, Ce, Hf, Ta, La, Ti, Sr, Ba, Nb, Mo and Cr, M^cIs at least one element selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr, p is 0.9-1.5, 0<x1≤0.4，0<y1 is not less than 0.4, z1 is not less than 0.1, q1 is not less than 0.1, and 0<x1+y1+z1≤0.4。

10. The positive electrode active material according to claim 1, further comprising a surface layer formed on an exterior circumference of the shell section,

wherein the surface layer includes a lithium composite transition metal oxide including at least one selected from the group consisting of Ni, Co, Mn, and Al, and a concentration of the transition metal in the surface layer is constant.

11. The positive electrode active material according to claim 1, wherein the positive electrode active material is composed of secondary particles in which primary particles of a lithium composite transition metal oxide are aggregated, and

a lithium ion diffusion path in the primary particle is formed toward the center of the secondary particle.

12. A method of preparing a positive electrode active material for a secondary battery, comprising:

co-precipitating a first precursor-forming solution to form a core, the first precursor-forming solution comprising: a Ni-containing raw material, a Co-containing raw material, and at least one selected from the group consisting of a Mn-containing raw material and an Al-containing raw material;

co-precipitating a second precursor-forming solution to form a shell portion, the second precursor-forming solution comprising: a Ni-containing raw material, a Co-containing raw material, and at least one selected from the group consisting of a Mn-containing raw material and an Al-containing raw material, wherein the concentration of the Ni-containing raw material is lower than that of the Ni-containing raw material of the first precursor-forming solution; and

forming a positive electrode active material precursor including the core section and the shell section formed around the core section, mixing the positive electrode active material precursor with a lithium source, and calcining the mixture, thereby forming a positive electrode active material including a lithium composite transition metal oxide,

wherein a ratio of a diameter of the core to a total diameter of particles of the positive electrode active material is 0.5 to 0.85, and

13. The method according to claim 12, wherein a co-precipitation time for forming the core is 0.5 to 0.85 times as long as an entire co-precipitation time for forming the positive electrode active material precursor.

14. The method of claim 12, wherein the co-precipitation time for forming the core is 13 to 32 hours.

15. The method according to claim 12, wherein the Ni content is 80 mol% or more of the total metal elements in the core portion.

16. The method of claim 12, wherein the shell portion has a concentration gradient such that the Ni concentration decreases from a beginning of the shell portion to an end of the shell portion.

17. The method according to claim 12, wherein a ratio of a thickness of the shell portion to a radius of particles of the positive electrode active material is 0.15 to 0.5.

18. A positive electrode for a secondary battery, comprising the positive electrode active material according to any one of claims 1 to 11.

19. A lithium secondary battery comprising the positive electrode according to claim 18.